The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2002-295952 filed on Oct. 9, 2002 the entire contents thereof are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a breather structure of an overhead-valve internal combustion engine, and more particularly to an improvement of a breather structure of an overhead-valve internal combustion engine in which a cylinder bore is formed in a cylinder block coupled to a crankcase which rotatably supports a crankshaft. A camshaft housing chamber, which houses a camshaft for performing open/close driving of an intake valve and an exhaust valve which are arranged in a cylinder head, is formed between the cylinder head which is coupled to the cylinder block. A driving force transmission chamber, which is arranged at the side of the cylinder bore is formed in the crankcase, the cylinder block and the cylinder head such that a driving force transmission member which transmits a rotational driving force of the crankshaft to the camshaft is housed in the driving force transmission chamber. A breather chamber, which is arranged below the camshaft housing chamber and at the side of the cylinder bore and the driving force transmission chamber, is formed such that the breather chamber extends between the cylinder block and the crankcase.
2. Description of Background Art
A breather structure is known as set forth, for example, in Japanese Unexamined Patent Publication 2000-220435. In this breather structure, a communication hole which functions as a breather inlet hole for introducing a blowby gas into the breather chamber and also as an oil discharge hole for discharging oil from the breather chamber through a lower portion of the inside of the breather chamber is formed in the cylinder block to permit communication of the lower portion of the breather chamber with the driving force transmission chamber.
However, in the above-mentioned conventional breather structure, there exists a possibility that when the blowby gas is introduced into the breather chamber from the driving force transmission chamber through the communication hole. Thus, the blowby gas impedes the discharge of oil from the communication hole to the driving force transmission chamber. Further, since the communication hole is present at the lower position of the crankcase, there exists the possibility that the rich blowby gas in which an oil mist generated in the crankcase is mixed is introduced into the breather chamber. In this case, it is difficult to acquire an excellent vapor-liquid separation performance.
The present invention is made in view of such circumstances and it is an object of the present invention to provide a breather structure of an overhead-valve internal combustion engine which performs the discharge of oil from a breather chamber smoothly and, at the same time, enhances the vapor-liquid separation performance.
To achieve the above-mentioned object, the present invention is directed to a breather structure of an overhead-valve internal combustion engine in which a cylinder bore is formed in a cylinder block coupled to a crankcase which rotatably supports a crankshaft. A camshaft housing chamber, which houses a camshaft for performing open/close driving of an intake valve and an exhaust valve which are arranged in a cylinder head, is formed between the cylinder head which is coupled to the cylinder block and the cylinder block. A driving force transmission chamber which is arranged at the side of the cylinder bore is formed in the crankcase, the cylinder block and the cylinder head such that a driving force transmission member which transmits a rotational driving force of the crankshaft to the camshaft is housed in the driving force transmission chamber. A breather chamber, which is arranged below the camshaft housing chamber and at the side of the cylinder bore and the driving force transmission chamber, is formed such that the breather chamber extends between the cylinder block and the crankcase. A breather inlet passage is provided which has an upper end thereof in communication with the inside of the camshaft housing chamber and a lower end thereof in communication with the breather chamber at a position corresponding to a mating face between the cylinder block and the crankcase and is arranged in the cylinder block such that the breather inlet passage extends vertically, and an oil discharge hole which is in communication with a lower portion of the inside of the breather chamber is arranged in the crankcase.
According to the present invention, since the breather inlet passage and the oil discharge hole are arranged at positions which are spaced apart from each other, there is no possibility that the discharge of the oil from the breather chamber is impeded by the blowby gas introduced into the breather chamber whereby the oil can be smoothly discharged. Further, the blowby gas ascends up to the camshaft housing chamber from the inside of the crankcase through the driving force transmission chamber. Thereafter, the blowby gas reverses the flow direction thereof and descends to at least the crankcase side of the breather chamber. Hence, the concentration of oil mist in the blowby gas which is introduced into the breather chamber is lowered whereby the vapor-liquid separation performance is enhanced.
The present invention provides a projecting portion which projects upwardly from a lower face of the camshaft housing chamber that is integrally formed on an upper portion of the cylinder block. The upper end of the breather inlet passage opens at an upper end of the projecting portion. Due to such a constitution, the oil which stays at the lower portion in the inside of the camshaft housing chamber is prevented from being introduced into the breather chamber through the breather inlet passage.
The present invention provides a cylinder gasket which partitions the breather chamber into a lower chamber at the crankcase side and an upper chamber at the cylinder block side to allows the flow of a blowby gas from the lower chamber to the upper chamber. The present invention is capable of separating a vapor-liquid mixture from the blowby gas inserted between the crankcase and the cylinder block. A communication hole is provided which allows a lower end of the breather inlet passage to communicate with the lower chamber that is formed in the cylinder gasket. Due to such a constitution, the blowby gas which is introduced to the breather chamber from the breather inlet passage is made to flow into the upper chamber from the lower chamber through the cylinder gasket. Furthermore, the cylinder gasket has a gas-liquid separation function and hence, it is possible to enhance the vapor-liquid performance without using special members.
The present invention provides a large number of small holes which make the lower chamber and the upper chamber communicate with each other that are formed in the cylinder gasket. Due to such a constitution, it is possible to efficiently perform the vapor-liquid separation with a simple structure.
Further, the present invention provides an oil discharge passage which is in communication with the oil discharge hole that has a lower end thereof in communication with the inside of the crankcase below an oil surface in the inside of the crankcase. Due to such a constitution, it is possible to prevent the rich blowby gas containing an oil mist generated in the inside of the crankcase from inversely flowing into the breather chamber from the oil discharge passage. At the same time, it is possible to ensure a return of the oil separated in the breather chamber to a lower portion of the inside of the crankcase.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.